JPWO2017159738A1 - Non-tempered steel bar - Google Patents

Abstract

Provided is a non-tempered steel bar that provides excellent cracking properties even if bainite is generated after hot forging. The non-tempered steel bar according to the present embodiment is mass%, C: 0.39 to 0.55%, Si: 0.10 to 1.00%, Mn: 0.50 to 1.50%, P: 0. 0.010 to 0.100%, S: 0.04 to 0.13%, Cr: 0.05 to 0.50%, V: 0.05 to 0.40%, Ti: 0.15 to 0.25 %, Al: 0.005 to 0.050%, N: 0.002 to 0.020%, the balance is composed of Fe and impurities, and has a chemical composition satisfying the formula (1). The number density of TiN having a circle-equivalent diameter of 20 μm or more is 0.3 to 4.0 pieces / mm 2. 0.60 ≦ C + 0.2Mn + 0.25Cr + 0.75V + 0.81Mo ≦ 1.00 (1) Here, the content (mass%) of the corresponding element is substituted into the element symbol in the formula (1).

Description

  The present invention relates to a steel bar, and more particularly to a steel bar (hereinafter referred to as a non-heat treated steel bar) used for a non-heat treated hot forged product.

  A connecting rod (hereinafter referred to as a connecting rod) used in an automobile engine or the like is an engine component that connects a piston and a crankshaft, and converts a reciprocating motion of the piston due to an explosion into a rotational motion of the crank.

  FIG. 1 is a front view of a conventional connecting rod. As shown in FIG. 1, the conventional connecting rod 1 includes a large end portion 100, a flange portion 200, and a small end portion 300. The large end portion 100 is disposed at one end of the flange portion 200, and the small end portion 300 is disposed at the other end of the flange portion 200. The large end 100 is connected to the crankpin. The small end 300 is connected to the piston.

  A conventional connecting rod 1 includes two parts (a cap 2 and a rod 3). These parts are usually manufactured by hot forging. One end portions of the cap 2 and the rod 3 correspond to the large end portion 100. Other parts than the one end of the rod 3 correspond to the flange 200 and the small end 300. The large end portion 100 and the small end portion 300 are formed by cutting. For this reason, the connecting rod 1 is required to have high machinability.

  The connecting rod 1 receives a load from peripheral members during engine operation. Recently, in order to further reduce fuel consumption, downsizing of the connecting rod 1 and improvement of in-cylinder pressure in the cylinder are required. Therefore, the connecting rod 1 is required to have an excellent yield strength that can cope with an explosion load transmitted from the piston even if the collar portion 200 is made thin. Furthermore, since the compressive load and the tensile load are repeatedly applied to the connecting rod, excellent fatigue strength is also required.

  In recent years, from the viewpoint of energy saving and cost reduction, non-tempered connecting rods in which tempering treatment (quenching and tempering) is omitted have begun to be adopted. Therefore, there is a demand for non-tempered steel that can provide sufficient yield strength, fatigue strength, and machinability without performing tempering after hot forging.

  By the way, as for the conventional connecting rod 1, the cap 2 and the rod 3 are manufactured separately as mentioned above. Therefore, a knock pin processing step is performed for positioning the cap 2 and the rod 3. Further, a cutting process is performed on the mating surface of the cap 2 and the rod 3. Therefore, cracking connecting rods that can omit these steps are beginning to spread.

  In the cracking connecting rod, after integrally forming the connecting rod, a jig is inserted into the hole of the large end portion 100, and the large end portion is broken by applying a stress to form two parts (corresponding to the cap 2 and the rod 3). To divide. Then, the two parts divided when attached to the crankshaft are joined. If the fracture surface of the large end 100 is a brittle fracture surface without deformation, the fracture surfaces of the cap 2 and the rod 3 can be combined and connected with bolts. Therefore, in this case, the knock pin machining process and the cutting process are omitted. As a result, the manufacturing cost is reduced.

  However, when mass producing cracking connecting rods, in the hot forging process, bainite may be partially generated in the hot forging product (cracking connecting rod) due to temperature variation of the heating furnace or processing heat generation. In this case, cracking properties are reduced. Specifically, since the toughness of bainite is high, if bainite is present in a hot forged product, a ductile fracture surface is likely to occur on the fracture surface after cracking. When a ductile fracture surface occurs, the large end portion is plastically deformed. Therefore, even if the fractured surfaces are matched, they are not perfectly aligned, and the inner diameter D of the large end portion 100 in FIG. 1 deviates from a desired numerical value. As a result, the crank connection portion (large end portion) is caused to come into contact with one another, which may cause vibration and noise when the vehicle is running.

  JP 2004-277817 A (Patent Document 1), JP 2011-195862 A (Patent Document 2), and International Publication No. 2009/107282 (Patent Document 3) propose steel with high cracking properties. .

The high strength non-tempered steel for fracture separation disclosed in Patent Document 1 is C: 0.2-0.6%, Si: 0.1-2%, Mn: 0.1-1. 5%, S: 0.03-0.2%, P: 0.02-0.15%, Cu: 0.03-1%, Ni: 0.03-1%, Cr: 0.05-1 %, V: 0.02-0.4%, Ti: 0.01-0.8%, s-Al: 0.005-0.045%, N: 0.008-0.035%, the balance is inevitable The composition is composed of mechanical impurities and Fe, and the structure is a ferrite pearlite structure. The maximum diameter of TiN inclusions in the steel is 5 μm or more, and the amount is 5 / mm ² or more in number density. In this document, it is described that moderate unevenness is formed on the fracture surface by the TiN, and the side slip on the bonding surface can be suppressed.

  Non-heat treated steel for hot forging disclosed in Patent Document 2 is C: 0.35-0.55%, Si: 0.15-0.40%, Mn: 0.50-1. 00%, P: 0.100% or less, S: 0.040 to 0.100%, Cr: 1.00% or less, V: 0.20 to 0.50%, Ca: 0.0005 to 0.0100 %, N: 0.0150% or less, the balance being Fe and inevitable impurities, 2Mn + 5Mo + Cr ≦ 3.1, C + Si / 5 + Mn / 10 + 10P + 5V ≧ 1.8, and Ceq = C + Si / 7 + Mn / 5 + Cr / 9 + V is 0.90 to 1.10. Furthermore, the hardness is HV330 or more, the yield ratio is 0.73 or more, and the structure is a ferrite pearlite structure having bainite of 10% or less. This document describes that the formation of bainite is suppressed by satisfying 2Mn + 5Mo + Cr ≦ 3.1, and that excellent cracking properties can be obtained by satisfying C + Si / 5 + Mn / 10 + 10P + 5V ≧ 1.8.

  The non-heat treated steel for hot forging disclosed in Patent Document 3 is mass%, C: more than 0.35% to 0.60%, Si: 0.50 to 2.50%, Mn: 0.20. -2.00%, P: 0.010-0.150%, S: 0.040-0.150%, V: 0.10-0.50%, Zr: 0.0005-0.0050%, Ca: 0.0005 to 0.0050%, N: 0.0020 to 0.0200%, Al: limited to less than 0.010%, the balance is substantially made of Fe and inevitable impurities, width The ratio of the number of MnS inclusions of 1 μm or more to the total MnS inclusions is 10% or less (including 0%), and the average aspect ratio of MnS inclusions is 10 or less. The bainite structure fraction is 3% or less (including 0%), and the remaining structure is a ferrite pearlite structure. Furthermore, in this document, it is described that the break separation property is improved by finely dispersing a large amount of MnS inclusions.

JP 2004-277817 A JP 2011-195862 A International Publication No. 2009/107282

  However, in Patent Document 1, when bainite is generated in a hot forged product, a ductile fracture surface may be generated on the fracture surface, and the inner diameter of the large end portion may be deformed to reduce cracking properties.

  In patent document 2, the production | generation of bainite in a hot forging product is permitted to some extent. However, in the case of the steel of Patent Document 2, a ductile fracture surface is generated on the fracture surface, and cracking properties may be reduced.

  In Patent Document 3, it is assumed that the microstructure of the hot forged product is mainly composed of ferrite and pearlite. Therefore, when bainite is generated in a hot forged product, cracking properties may be deteriorated.

  An object of the present invention is to provide a non-tempered steel bar which has high machinability, yield strength and fatigue strength, and can obtain excellent cracking properties even if bainite is generated after hot forging.

The non-tempered steel bar according to the present embodiment is mass%, C: 0.39 to 0.55%, Si: 0.10 to 1.00%, Mn: 0.50 to 1.50%, P: 0. 0.010 to 0.100%, S: 0.040 to 0.130%, Cr: 0.05 to 0.50%, V: 0.05 to 0.40%, Ti: 0.15 to 0.25 %, Al: 0.005 to 0.050%, N: 0.002 to 0.020%, Cu: 0 to 0.40%, Ni: 0 to 0.30%, Mo: 0 to 0.10% Pb: 0 to 0.30%, Te: 0 to 0.3000%, Ca: 0 to 0.0100%, and Bi: 0 to 0.3000%, the balance being Fe and impurities, The number density of TiN having a chemical composition satisfying the formula (1) and having a circle-equivalent diameter of 20 μm or more in steel is 0.3 to 4.0 pieces / mm ² .
0.60 ≦ C + 0.2Mn + 0.25Cr + 0.75V + 0.81Mo ≦ 1.00 (1)
Here, the content (mass%) of the corresponding element is substituted for the element symbol in the formula (1).

  The non-heat treated steel bar according to the present embodiment has high machinability, yield strength and fatigue strength, and excellent cracking properties can be obtained even if bainite is generated after hot forging.

FIG. 1 is a front view of a conventional connecting rod. FIG. 2A is a plan view of a test piece used in a cracking property evaluation test in Examples. FIG. 2B is a cross-sectional view of the test piece shown in FIG. 2A. FIG. 2C is a plan view of the test piece showing a state in which the test piece of FIG. 2A is broken and separated. FIG. 2D is a plan view of the test piece showing a state in which the test piece of FIG. 2C is fastened with a bolt.

  The present inventors investigated and examined the strength (yield strength and fatigue strength), machinability, and crackability after hot forging of non-tempered steel bars. As a result, the present inventors obtained the following knowledge.

  (1) Yield strength, fatigue strength, and machinability are contradictory mechanical properties. If the chemical components can be adjusted appropriately, these mechanical properties can be achieved.

  It is defined as fn1 = C + 0.2Mn + 0.25Cr + 0.75V + 0.81Mo. fn1 is an index of yield strength and shows a positive correlation with the yield strength. If fn1 is less than 0.60, the yield strength of steel is too low. If fn1 is higher than 1.00, the tensile strength of the steel becomes too high, and the machinability of the steel decreases. If fn1 is 0.60 to 1.00, excellent yield strength and machinability can be obtained.

  (2) Even if bainite is generated in the microstructure of a hot forged product, excellent cracking properties can be obtained while maintaining hot workability by keeping the number density of coarse TiN within an appropriate range. .

  In the solidification process of the molten steel by continuous casting, Ti forms Ti nitride (TiN), Ti sulfide and Ti carbon sulfide. Among these, TiN remains without being dissolved in the heating step before hot forging. Therefore, such TiN remains in the hot forged product. The remaining TiN becomes a starting point of fracture at a plurality of locations during cracking, and a sharp initial crack is generated at the interface between TiN and the matrix. Since the tip of a sharp crack is in a state of strong plastic restraint, brittle fracture is likely to occur. A brittle fracture surface is obtained by combining a crack that has developed brittlely from an initial crack with a crack that has occurred from adjacent TiN. Therefore, even in a microstructure containing bainite having high toughness, the above-described initial crack is generated by TiN, thereby causing brittle crack growth, the fracture surface becomes a brittle fracture surface, and the ductile fracture surface is suppressed. As a result, excellent cracking properties can be obtained.

In order to obtain such an effect, a larger amount of TiN is preferable. Specifically, if the number density of TiN having an equivalent circle diameter of 20 μm or more (hereinafter referred to as coarse TiN) is less than 0.3 / mm ² , sufficient cracking properties cannot be obtained. On the other hand, if the number density of coarse TiN exceeds 4.0 pieces / mm ² , excellent cracking properties can be obtained, but hot workability decreases. If the number density of coarse TiN is 0.3 to 4.0 pieces / mm ² , excellent cracking properties can be obtained while maintaining hot workability even when bainite is generated by hot forging.

The non-tempered steel bar of this embodiment completed by the above knowledge is mass%, C: 0.39-0.55%, Si: 0.10-1.00%, Mn: 0.50-1. 50%, P: 0.010 to 0.100%, S: 0.040 to 0.130%, Cr: 0.05 to 0.50%, V: 0.05 to 0.40%, Ti: 0 15 to 0.25%, Al: 0.005 to 0.050%, N: 0.002 to 0.020%, Cu: 0 to 0.40%, Ni: 0 to 0.30%, Mo: 0 to 0.10%, Pb: 0 to 0.30%, Te: 0 to 0.3000%, Ca: 0 to 0.0100%, and Bi: 0 to 0.3000%, the balance being Fe and impurities, having a chemical composition satisfying the formula (1), the number density of TiN having 20μm or more circle-equivalent diameter in the steel 0.3 to 4.0 pieces / mm ² A.
0.60 ≦ C + 0.2Mn + 0.25Cr + 0.75V + 0.81Mo ≦ 1.00 (1)
Here, the content (mass%) of the corresponding element is substituted for the element symbol in the formula (1).

  The chemical composition is one or two selected from the group consisting of Cu: 0.01 to 0.40%, Ni: 0.01 to 0.30%, and Mo: 0.01 to 0.10%. It may contain seeds or more. The chemical composition is Pb: 0.05 to 0.30%, Te: 0.0003 to 0.3000%, Ca: 0.0003 to 0.0100%, and Bi: 0.0003 to 0.3000%. You may contain 1 type, or 2 or more types selected from the group which consists of.

  Hereinafter, the non-heat treated steel bar of this embodiment will be described in detail. “%” Of the content of each element means “mass%”.

[Chemical composition]
The chemical composition of the non-tempered steel bar according to the present embodiment contains the following elements.

C: 0.39 to 0.55%
Carbon (C) increases the yield strength and fatigue strength of steel. If the C content is too low, this effect cannot be obtained. On the other hand, if the C content is too high, the machinability decreases. Therefore, the C content is 0.39 to 0.55%. The minimum with preferable C content is 0.40%, More preferably, it is 0.41%, More preferably, it is 0.42%. The upper limit with preferable C content is 0.54%, More preferably, it is 0.53%, More preferably, it is 0.52%.

Si: 0.10 to 1.00%
Silicon (Si) deoxidizes steel. Si further dissolves in the steel to increase the fatigue strength of the steel. If the Si content is too low, these effects cannot be obtained. On the other hand, if the Si content is too high, the above effect is saturated. If the Si content is too high, the hot workability of the steel further decreases, and the manufacturing cost of the steel bar increases. Therefore, the Si content is 0.10 to 1.00%. The minimum with preferable Si content is 0.11%, More preferably, it is 0.12%, More preferably, it is 0.15%. The upper limit with preferable Si content is 0.99%, More preferably, it is 0.95%, More preferably, it is 0.90%, More preferably, it is 0.89%.

Mn: 0.50 to 1.50%
Manganese (Mn) deoxidizes steel. Mn further increases the yield strength and fatigue strength of the steel. If the Mn content is too low, these effects cannot be obtained. On the other hand, if the Mn content is too high, the hot workability of the steel decreases. Therefore, the Mn content is 0.50 to 1.50%. The minimum with preferable Mn content is 0.51%, More preferably, it is 0.55%, More preferably, it is 0.60%. The upper limit with preferable Mn content is 1.49%, More preferably, it is 1.45%, More preferably, it is 1.40%.

P: 0.010-0.100%
Phosphorus (P) segregates at the grain boundaries and embrittles the steel. Therefore, the fracture surface of the cracking connecting rod after fracture splitting becomes brittle. As a result, the inner diameter deformation amount of the large end portion of the cracking connecting rod after the fracture split becomes small. If the P content is too low, this effect cannot be obtained. On the other hand, if P content is too high, the hot workability of steel will fall. Therefore, the P content is 0.010 to 0.100%. The minimum with preferable P content is 0.011%, More preferably, it is 0.015%, More preferably, it is 0.020%. The upper limit with preferable P content is 0.090%, More preferably, it is 0.080%, More preferably, it is 0.070%.

S: 0.040 to 0.130%
Sulfur (S) combines with Mn and Ti to form sulfides and enhances the machinability of steel. If the S content is too low, this effect cannot be obtained. On the other hand, if the S content is too high, the hot workability of the steel decreases. Therefore, the S content is 0.040 to 0.130%. The minimum with preferable S content is 0.041%, More preferably, it is 0.045%, More preferably, it is 0.050%. The upper limit with preferable S content is 0.129%, More preferably, it is 0.125%, More preferably, it is 0.120%.

Cr: 0.05 to 0.50%
Chromium (Cr) increases the yield strength and fatigue strength of steel. If the Cr content is too low, this effect cannot be obtained. On the other hand, if the Cr content is too high, the hardness of the steel material increases and the machinability decreases. Furthermore, if the Cr content is too high, the manufacturing cost increases. Therefore, the Cr content is 0.05 to 0.50%. The preferred lower limit of the Cr content is 0.10%, more preferably 0.12%, more preferably 0.15%, and the preferred upper limit of the Cr content is 0.49%, more preferably 0.45%, more preferably 0.40%.

V: 0.05 to 0.40%
Vanadium (V) precipitates as carbide in the ferrite during the cooling process after hot forging, and increases the yield strength and fatigue strength of the steel. V is further contained together with Ti to enhance the cracking property of steel. If the V content is too low, these effects cannot be obtained. On the other hand, if the V content is too high, not only the production cost of steel becomes extremely high, but also the machinability decreases. Therefore, the V content is 0.05 to 0.40%. The minimum with preferable V content is 0.06%, More preferably, it is 0.07%, More preferably, it is 0.10%. The upper limit with preferable V content is 0.39%, More preferably, it is 0.35%, More preferably, it is 0.32%.

Ti: 0.15-0.25%
Titanium (Ti) forms TiN in the solidification process of continuous casting and improves cracking properties. More specifically, Ti forms TiN, Ti sulfide and Ti carbon sulfide in the solidification process of the molten steel by continuous casting. The TiN formed at this time does not dissolve in the subsequent heating step before hot forging, and improves the cracking properties by satisfying the conditions of size and number density described later.

  Further, Ti precipitates as carbide in the ferrite together with V in the cooling process after hot forging, and increases the fatigue strength of the steel. Ti further produces sulfides or carbosulfides to enhance the machinability of the steel. More specifically, when the non-tempered steel bar is heated before hot forging, a part of Ti in the Ti sulfide and Ti carbon sulfide in the steel is dissolved. Furthermore, when the steel material is allowed to cool to the air after hot forging, a part of Ti remains in solid solution until the ferrite transformation is started. When the ferrite transformation is started, solute Ti precipitates as carbide together with V in the ferrite, increasing the fatigue strength of the steel. Furthermore, Ti sulfide and carbosulfide which remain in the steel without being dissolved, increase the machinability of the steel.

  If the Ti content is too low, these effects cannot be obtained. On the other hand, if the Ti content is too high, the hot workability decreases. Therefore, the Ti content is 0.15 to 0.25%. The minimum with preferable Ti content is more than 0.15%, More preferably, it is 0.16%. The upper limit with preferable Ti content is 0.24%, More preferably, it is 0.22%.

Al: 0.005 to 0.050%
Aluminum (Al) deoxidizes steel. If the Al content is too low, this effect cannot be obtained. On the other hand, if the Al content is too high, the above effect is saturated. If the Al content is too high, the hot workability of the steel is further lowered, and the production cost of the steel material is increased. Therefore, the Al content is 0.005 to 0.050%. The minimum with preferable Al content is 0.020%. The upper limit with preferable Al content is 0.040%. In the non-heat treated steel bar of this embodiment, the Al content means acid-soluble Al (so-called “sol.Al”).

N: 0.002 to 0.020%
Nitrogen (N) combines with Ti to form TiN and enhances cracking properties. If the N content is too low, this effect cannot be obtained. On the other hand, if the N content is too high, the hot workability of the steel decreases. Therefore, the N content is 0.002 to 0.020%. The minimum with preferable N content is 0.003%, More preferably, it is 0.004%, More preferably, it is 0.005%. The upper limit with preferable N content is 0.019%, More preferably, it is 0.018%, More preferably, it is 0.017%.

  The balance of the chemical composition of the non-tempered steel bar according to the present embodiment is composed of Fe and impurities. Here, the impurities are mixed from the ore as a raw material, scrap, or the manufacturing environment when industrially producing the non-heat treated steel bar, and have an adverse effect on the non-heat treated steel bar of the present embodiment. It means that it is allowed in the range that does not give.

  The chemical composition of the non-tempered steel bar according to the present embodiment may further include one or more selected from the group consisting of Cu, Ni, and Mo instead of part of Fe. These elements are arbitrary elements, and all increase the fatigue strength of steel.

Cu: 0 to 0.40%
Copper (Cu) is an optional element and may not be contained. When contained, Cu dissolves in the steel and increases the fatigue strength of the steel. However, if the Cu content is too high, not only the production cost of steel increases, but also the machinability decreases. Therefore, the Cu content is 0 to 0.40%. The minimum with preferable Cu content is 0.01%, More preferably, it is 0.05%, More preferably, it is 0.10%. The upper limit with preferable Cu content is 0.39%, More preferably, it is 0.35%, More preferably, it is 0.30%.

Ni: 0 to 0.30%
Nickel (Ni) is an optional element and may not be contained. When contained, Ni dissolves in the steel and increases the fatigue strength of the steel. However, if the Ni content is too high, not only the manufacturing cost increases, but also the toughness increases, and a ductile fracture surface is generated on the fracture surface after fracture separation, and the cracking property decreases. Therefore, the Ni content is 0 to 0.30%. The minimum with preferable Ni content is 0.01%, More preferably, it is 0.02%, More preferably, it is 0.05%. The upper limit with preferable Ni content is 0.29%, More preferably, it is 0.28%, More preferably, it is 0.25%.

Mo: 0 to 0.10%
Molybdenum (Mo) is an optional element and may not be contained. When contained, Mo forms carbides in the steel to increase the yield strength and fatigue strength of the steel. However, if the Mo content is too high, the hardness of the steel material increases and the machinability decreases. Furthermore, if the Mo content is too high, the manufacturing cost increases. Therefore, the Mo content is 0 to 0.10%. The minimum with preferable Mo content is 0.01%, More preferably, it is 0.02%, More preferably, it is 0.05%. The upper limit with preferable Mo content is 0.09%, More preferably, it is 0.08%, More preferably, it is 0.07%.

  The chemical composition of the non-tempered steel bar according to the present embodiment may further include one or more selected from the group consisting of Pb, Te, Ca and Bi instead of a part of Fe. These elements are arbitrary elements, and all enhance the machinability of steel.

Pb: 0 to 0.30%
Lead (Pb) is an optional element and may not be contained. When contained, Pb increases the machinability of the steel. However, if the Pb content is too high, the hot workability of the steel decreases. Therefore, the Pb content is 0 to 0.30%. The minimum with preferable Pb content is 0.05%, More preferably, it is 0.10%. The upper limit with preferable Pb content is 0.29%, More preferably, it is 0.25%, More preferably, it is 0.20%.

Te: 0 to 0.3000%
Tellurium (Te) is an optional element and may not be contained. When contained, Te increases the machinability of the steel. However, if the Te content is too high, the hot workability of the steel decreases. Therefore, the Te content is 0 to 0.3000%. The minimum with preferable Te content is 0.0003%, More preferably, it is 0.0005%, More preferably, it is 0.0010%. The upper limit with preferable Te content is 0.2900%, More preferably, it is 0.2500%, More preferably, it is 0.2000%.

Ca: 0 to 0.0100%
Calcium (Ca) is an optional element and may not be contained. When contained, Ca increases the machinability of steel. However, if the Ca content is too high, the hot workability of the steel decreases. Therefore, the Ca content is 0 to 0.0100%. The minimum with preferable Ca content is 0.0003%, More preferably, it is 0.0005%, More preferably, it is 0.00100%. The upper limit with preferable Ca content is 0.0090%, More preferably, it is 0.0080%, More preferably, it is 0.0050%.

Bi: 0 to 0.3000%
Bismuth (Bi) is an optional element and may not be contained. When contained, Bi increases the machinability of the steel. However, if the Bi content is too high, the hot workability of the steel decreases. Therefore, the Bi content is 0 to 0.3000%. The minimum with preferable Bi content is 0.0003%, More preferably, it is 0.0005%, More preferably, it is 0.0010%. The upper limit with preferable Bi content is 0.2900%, More preferably, it is 0.2000%, More preferably, it is 0.1000%.

[Regarding Formula (1)]
The chemical composition of the non-tempered steel bar of this embodiment further satisfies the formula (1).
0.60 ≦ C + 0.2Mn + 0.25Cr + 0.75V + 0.81Mo ≦ 1.00 (1)
Here, the content (mass%) of the corresponding element is substituted for the element symbol in the formula (1).

  If fn1 (= C + 0.2Mn + 0.25Cr + 0.75V + 0.81Mo) is less than 0.60, the yield strength of the steel is too low. If fn1 is higher than 1.00, the strength of the steel becomes too high, and the machinability of the steel decreases. If fn1 is 0.60 to 1.00, excellent yield strength and machinability can be obtained in a non-tempered steel bar. The minimum with preferable fn1 is 0.61, More preferably, it is 0.63, More preferably, it is 0.65. The upper limit with preferable fn1 is 0.99, More preferably, it is 0.98, More preferably, it is 0.95.

[Microstructure]
In the case of the above chemical composition, the microstructure of the non-heat treated steel bar is mainly composed of ferrite and pearlite. Specifically, in the non-heat treated steel bar having the above chemical composition, the total area ratio of ferrite and pearlite in the microstructure is 65% or more. When the total area ratio of ferrite and pearlite is not 100%, the balance of the matrix structure is bainite. A preferable lower limit of the total area ratio of ferrite and pearlite is 70%, more preferably 75%, still more preferably 80% or more, and most preferably 100%. The upper limit of the area ratio of bainite is preferably 30%, more preferably 25%, still more preferably 20%, and most preferably 0%.

  The area ratio of bainite in the microstructure can be measured by the following method. Ten samples are collected from an arbitrary R / 2 part of the non-heat treated steel bar (the central part of the line segment (radius) connecting the central axis of the steel bar and the outer peripheral surface). Among the collected samples, a surface perpendicular to the central axis of the non-heat treated steel bar is taken as an observation surface. After the observation surface is polished, it is etched with 3% nitric acid alcohol (nitral etchant). The etched observation surface is observed with a 200 × optical microscope, and photographic images with arbitrary five fields of view are generated.

In each field of view, each phase such as ferrite, pearlite, and bainite has a different contrast for each phase. Therefore, each phase is specified based on the contrast. Among the identified phases, the area of bainite (μm ² ) in each visual field is obtained. The ratio of the area of bainite in all visual fields to the total area of all visual fields (5 visual fields × 10) is defined as the area ratio (%) of bainite.

[Number density of coarse TiN]
In the non-heat treated steel bar according to the present embodiment, the number density of TiN (hereinafter referred to as coarse TiN) having an equivalent circle diameter of 20 μm or more is 0.3 to 4.0 pieces / mm ² . Here, in this specification, TiN means inclusions in which the total content of Ti and N in inclusions is 70% or more by mass%.

  The non-tempered steel bar of this embodiment is manufactured into a cracking connecting rod by hot forging. When the steel temperature at the time of hot forging becomes higher than 1300 ° C due to variation in the heating temperature during operation, bainite is generated in the microstructure of the hot forged product (cracking connecting rod) together with ferrite and pearlite. There is a case. In this case, in the chemical composition, the area ratio of bainite that can be generated is, for example, 5 to 30%.

  Bainite has higher toughness than ferrite and pearlite. Therefore, when two parts (a cap and a rod) are manufactured by breaking the large end portion of the cracking connecting rod, the broken portion is plastically deformed, and a ductile fracture surface is generated on the fracture surface. That is, cracking properties are reduced.

Even if bainite is generated in the microstructure by hot forging, the non-tempered steel bar of this embodiment maintains excellent cracking properties by keeping the number density of the coarse TiN within an appropriate range. If the number density of coarse TiN is less than 0.3 pieces / mm ² , sufficient cracking properties cannot be obtained. On the other hand, if the number density of coarse TiN exceeds 4.0 pieces / mm ² , excellent cracking properties can be obtained, but hot workability decreases. If the number density of coarse TiN is 0.3 to 4.0 pieces / mm ² , excellent cracking property is maintained while maintaining hot workability even if hot forging is performed under the condition that bainite is generated. can get.

The preferable lower limit of the number density of coarse TiN for further improving the cracking property is 0.4 / mm ² , more preferably 0.5 / mm ² . The upper limit of the number density of coarse TiN for further improving hot workability is 3.9 / mm ² , and more preferably 3.8 / mm ² .

The number density of coarse TiN can be measured by the following method. A sample is taken from R / 2 part of the steel bar. Of the surface of the sample, the surface corresponding to the cross section (longitudinal cross section) including the axial direction of the steel bar is taken as the observation surface. The observation surface is not corroded, and is directly observed with a 200 × optical microscope, and a photographic image is generated with an arbitrary 100 fields of view. The total area of 100 views is 11.9 mm ² . The total content of Ti and N in each field inclusion and precipitate is analyzed using an electron beam microanalyzer (EPMA) and identified as TiN in each field. Using the photographic image of each field of view, the area of each identified TiN is obtained, and the equivalent circle diameter is calculated from the obtained area. TiN having an equivalent circle diameter of 20 μm or more is specified as coarse TiN, and the total number of coarse TiN is obtained. The value obtained by dividing the total number of coarse TiNs obtained by the total area of 100 fields is defined as the number density (pieces / mm ² ) of coarse TiN.

[Production method]
An example of the manufacturing method of the above-mentioned non-tempered steel bar will be described. This manufacturing method includes a casting process and a hot rolling process.

[Casting process]
Molten steel satisfying the above-described chemical composition and formula (1) is produced by a well-known method. Using molten steel, a slab (slab or bloom) is produced by a continuous casting method.

  In order to make the number density of coarse TiN in the above range, in the casting process, continuous casting is performed so as to satisfy the following conditions.

Superheat degree ΔT: 30-50 ° C.
The difference between the molten steel temperature and the TLL (liquidus temperature) in the tundish arranged on the continuous casting machine is defined as the degree of superheat ΔT (° C.). If ΔT is less than 30 ° C., the amount of TiN crystallized becomes insufficient. On the other hand, if the degree of superheat ΔT (° C.) exceeds 50 ° C., coarse TiN is excessively generated, and the number density of coarse TiN exceeds 4.0 pieces / mm ² . If superheat degree (DELTA) T is 30-50 degreeC, the amount of crystallization of coarse TiN can be made into an appropriate range, stabilizing operation.

Cross section of slab: One side length is 300 mm or more Casting speed Vc: 0.2 to 0.8 m / min
If the cooling rate in the solidification process of the slab is too fast, TiN crystallization and aggregation become insufficient. On the other hand, if the cooling rate is too slow, TiN aggregates excessively and the number density of coarse TiN exceeds 4.0 / mm ² .

If one side of the cross section (rectangle) of the slab is 300 mm or more and the casting speed Vc is 0.2 to 0.8 m / min, TiN is sufficiently crystallized and agglomerated, so that coarse TiN The number density is 0.3 pieces / mm ² or more.

  The specific water amount is not particularly limited, and may be a known specific water amount. Preferably, the specific water amount is preferably as low as possible so that the slab does not bulge. A preferred specific water amount is, for example, 5 L / kg or less.

[Hot working process]
In the hot working process, hot working is performed on the slab produced in the casting process to produce a steel bar. The hot working process includes, for example, a rough rolling process and a finish rolling process.

[Rough rolling process]
A billet is manufactured by hot rolling a slab or an ingot. For example, the hot rolling is performed using a block rolling mill and a continuous rolling mill in which a plurality of stands are arranged in a line and each stand has a plurality of rolls.

[Finishing rolling process]
Steel bars are manufactured using billets. Specifically, the billet is heated in a heating furnace (heating process). After heating, the billet is hot-rolled (finish rolling) using a continuous rolling mill to produce non-tempered steel bar (finish rolling process). Hereinafter, each step will be described.

[Heating process]
In the heating step, the billet is preferably heated at a heating temperature of 1000 to 1300 ° C. for 30 minutes or more. If the heating temperature is too low, TiN in the billet will not easily aggregate. Therefore, the fine TiN existing in the billet is not agglomerated and is inherited even after hot rolling, and there are many fine Ti nitrides in the steel bar. In this case, coarse TiN in the steel is reduced. On the other hand, if the heating temperature is too high, Ti nitride aggregates excessively during heating. If the heating temperature during finish rolling is 1000 to 1300 ° C., the number density of coarse TiN is stable and in an appropriate range (0.3 to 4.0 pieces / mm ² ) on the assumption that the above casting conditions are satisfied. )

[Hot rolling process]
Using a finish rolling mill, the billet after heating is finish-rolled (hot-rolled) by a well-known method to produce a non-tempered steel bar. The finish rolling mill has a plurality of stands arranged in a row, and each stand has a plurality of rolls (roll groups) arranged around a pass line. The roll group of each stand forms a hole mold, and when the billet passes through the hole mold, it is rolled down to produce a steel bar.

The area reduction rate in the continuous rolling mill is preferably 70% or more. Here, the area reduction rate is defined by the following equation.
Area reduction ratio = (cross-sectional area of billet before finish rolling−cross-sectional area of non-heat treated steel bar after finish rolling) / cross-sectional area of billet before finish rolling × 100

  The above-mentioned non-tempered steel bar is manufactured by the above manufacturing process.

[Method for manufacturing hot forged products]
As an example of a method for producing a hot forged product using the above-mentioned non-heat treated steel bar, a method for producing a cracking connecting rod will be described. First, a steel material is heated in a high frequency induction heating furnace. In this case, a preferable heating temperature is 1000 to 1300 ° C., and a preferable heating time is 10 to 15 minutes. Since the heating time is short, the form of Ti nitride in the steel bar is not particularly changed. Hot forging is performed on the heated steel bar to produce a cracking connecting rod. Preferably, the degree of processing during hot forging is 0.22 or more. Here, the working degree is the maximum value of the logarithmic strain generated in the portion excluding burrs in the forging process.

  The cracking connecting rod after hot forging is allowed to cool to room temperature. The connecting rod large end has a small cross-sectional area, so the cooling rate is fast. Therefore, the form of TiN is not particularly changed during cooling. Machining is performed as necessary on the cracking connecting rod after cooling. The cracking connecting rod is manufactured by the above process.

[Microstructure of hot forged products]
The microstructure of the manufactured hot forged product (cracking connecting rod) is mainly composed of ferrite and pearlite. Preferably, the microstructure has a total area ratio of ferrite and pearlite of 100%. However, if the heating temperature of the steel bar during hot forging exceeds 1300 ° C., the microstructure of the manufactured cracking connecting rod can include bainite.

  In the microstructure of the cracking connecting rod manufactured by hot forging using the non-heat treated steel bar, the total area ratio of ferrite and pearlite is 65% or more. When the total area ratio of ferrite and pearlite is not 100%, the balance of the matrix structure is bainite. A preferable lower limit of the total area ratio of ferrite and pearlite is 70%, more preferably 75%, still more preferably 80% or more, and most preferably 100%. The upper limit of the area ratio of bainite is preferably 30%, more preferably 25%, still more preferably 20%, and most preferably 0%. An example of the area ratio of bainite is 5 to 30%.

When bainite is included in the microstructure, when the large end is fractured and divided into two parts (cap and rod), the fractured part is plastically deformed and a part of the fracture surface tends to become a ductile fracture surface, and cracking properties Is prone to decline. However, in the non-tempered steel bar of the present embodiment, the number density of coarse TiN in the steel is 0.3 to 4.0 pieces / mm ² , so that the fracture surface tends to be a brittle fracture surface and has excellent cracking properties. Can be maintained.

  The area ratio of bainite in the microstructure in the hot forged product can be measured by the following method. Ten samples are taken from any part of the hot forging. For each sample collected, the phase of the microstructure is identified and the area ratio of bainite is obtained by the same method as the microstructure observation in the non-heat treated steel bar.

  In the above description, a cracking connecting rod has been described as an example of a method for producing a forged product. However, the non-heat treated steel bar of this embodiment is not limited to cracking connecting rod applications. The non-tempered steel bar of this embodiment can be widely applied to forged products.

  Moreover, the manufacturing method of a non-tempered steel bar will not be limited to the said manufacturing method, if the number density of coarse TiN can be made into the said range.

  Molten steel having the chemical composition shown in Table 1 was produced.

  Referring to Table 1, the chemical compositions of Test Nos. E-1 to E-45, C-9, C-10, C-12, and C-13 were appropriate and satisfied Formula (1). On the other hand, for test numbers C-1 to C-8 and C-11, the content of any element in the chemical composition was inappropriate or did not satisfy the formula (1). Further, the chemical composition of test number C-11 was within the range of the chemical composition of steel described in Patent Document 1.

  Molten steel of each test number was produced in a 70 ton converter. Using a continuous casting machine, a slab (bloom) was produced from molten steel by a continuous casting method. The cross section of the bloom was 300 mm × 400 mm. In each test number, the molten steel temperature (° C.) in the tundish was measured, and the degree of superheat ΔT (° C.), which is the difference between the molten steel temperature and TLL (liquidus temperature), was obtained. Further, in each test number, casting was performed at a casting speed Vc (m / min) shown in Table 2. In any of the test numbers, the specific water amount was 5 L / kg or less.

  The billet was manufactured by hot rolling the manufactured slab. The billet was heated at 1150 ° C. for 35 minutes and then subjected to finish rolling using a finish rolling mill to produce a steel bar having a diameter of 40 mm.

[Manufacture of heat forged simulated products]
The steel bar was cut in a direction perpendicular to the longitudinal direction, and a specimen having a diameter of 40 mm and a length of 100 mm was collected. The specimen was heated and held at 1300 ° C. for 5 minutes. Immediately after the heating, 90% hot compression was performed in the axial direction to form a disk shape, thereby producing a hot forging simulated product (referred to as a hot forging simulated product). The heat forged simulated product after molding was allowed to cool in the atmosphere. The temperature of the test pieces measured using a radiation thermometer during compression was 1350 ° C.

[Evaluation test]
The following evaluation tests were carried out using the test materials and the heat forged simulated products.

[Number density measurement of coarse TiN]
Samples were taken from R / 2 parts of each test material. Of the surface of the sample, the surface corresponding to the cross section (longitudinal cross section) including the axial direction of the specimen was used as the observation surface. The observation surface was not corroded, and was directly observed with a 200 × optical microscope, and a photographic image was generated with an arbitrary 100 fields of view. The total area of 100 fields of view was 11.9 mm ² . TiN was identified by the method described above, and the number density (pieces / mm ² ) of coarse TiN was determined. The obtained number density is shown in Table 2.

[Microstructure observation]
A microstructure observation test was carried out using each heat forged simulated product. Specifically, a sample containing R / 2 part was collected from the longitudinal section of the thermal forging simulated product. The surface perpendicular to the central axis of the non-heat treated steel bar was used as the observation surface. After the observation surface was polished, it was etched with 3% nitric alcohol (nitral corrosive solution). The etched observation surface was observed with a 200-fold optical microscope, and the area ratio (%) of bainite was determined by the method described above. Table 2 shows the obtained area ratio of bainite.

[Hot workability evaluation]
50 heat forged simulated products were manufactured for each test number. The presence or absence of cracks on the surface of the simulated thermal forging product after production was visually confirmed. When the number of occurrences of cracks is 0 out of 50, the evaluation is “A”, the case of 1 is evaluation “B”, and the case of 2 to 3 is evaluation “C”. When it was, the evaluation was “x”. In the case of evaluation “A” to “C”, it was determined that sufficient hot workability was obtained, and in the case of evaluation “x”, it was determined that the hot workability was low. The evaluation results are shown in Table 2.

[Cracking evaluation]
A test piece 10 simulating the large end portion of the connecting rod shown in FIG. 2A was manufactured by machining from each heat forged simulated product. The length of one side of the test piece 10 was 80 mm, and the thickness was 10 mm. A hole (through hole) 11 was formed in the center of the test piece 10. The diameter of the hole 11 was 60 mm, and the center thereof was coaxial with the center of the test piece 10. As shown in FIG. 2A, V-shaped notches M were processed at two locations corresponding to the end points of the diameter of the periphery of the hole 11. The depth of the notch M was 1 mm, the tip R was 0.1 mm, and the opening angle was 60 °.

  The jig 12 was fitted into the hole 11. The jig 12 was composed of a pair of semicircular members, and when the two were combined, a disc having a diameter corresponding to the inner diameter of the hole 11 was obtained. A hole 14 for driving the wedge 13 was formed at the center of the jig 12 (see FIG. 2B).

  After fitting the jig 12 into the hole 11, the wedge 13 was driven to break the test piece 10 into two members 10A and 10B at room temperature (25 ° C.) (see FIG. 2C).

  Bolt hole processing was performed near both side surfaces of the members 10A and 10B, and the members 10A and 10B were fastened with the bolts 15 shown in FIG. 2D. The diameter D0 (see FIG. 2A) of the hole 11 of the test piece 10 before breaking separation and the diameter D1 (FIG. 2D) of the hole 11 of the test piece 10 after breaking separation and fastening the bolt 15 are measured. The difference was defined as the inner diameter deformation amount ΔD (= D1−D0, unit is μm).

  When the inner diameter deformation amount ΔD is 0 to 30 μm, the evaluation is “A”, 31 to 50 μm is the evaluation “B”, and 51 to 80 is the evaluation “C”. When the inner diameter deformation amount ΔD is 81 μm or more, the evaluation is “x”. In the case of evaluations “A” to “C”, it was judged that sufficient cracking properties were obtained. In the case of evaluation “x”, it was judged that the cracking property was low.

[Yield strength evaluation]
Two JIS 14A specimens were collected from R / 2 part of each thermal forging simulated product. Using the collected specimens, a tensile test was carried out at room temperature (25 ° C.) in the atmosphere to determine the average yield strength YS (MPa) of the two.

  A case where the yield strength was 1000 to 801 MPa was evaluated as “A”, a case where the yield strength was 800 to 601 MPa was evaluated as “B”, and a case where the yield strength was 600 to 401 MPa was evaluated as “C”. The case where the yield strength was 400 MPa or less was evaluated as “x”.

  In the case of evaluations “A” to “C”, it was judged that sufficient yield strength was obtained. In the case of evaluation “x”, it was judged that the yield strength was low.

[Fatigue strength evaluation]
A JIS No. 14A test piece was collected from R / 2 part of each thermal forging simulated product. Using the collected specimens, a double-sided fatigue test with a sine wave at phase 0 (MPa) was performed at room temperature (25 ° C.) in the atmosphere. The maximum stress that did not break at a repetition number of 10 ⁷ times was defined as fatigue strength (MPa). The frequency was 15 Hz.

  The case where the fatigue strength was 500 to 451 MPa was evaluated as “S”, the case where 450 to 401 MPa was evaluated as “A”, the case where 400 to 351 MPa was evaluated as “B”, and the case where 350 to 301 MPa was evaluated as “C”. The case where the fatigue strength was 300 MPa or less was evaluated as “x”.

  In the case of evaluations “S” and “A” to “C”, it was judged that sufficient fatigue strength was obtained. In the case of evaluation “x”, it was judged that the fatigue strength was low.

[Machinability evaluation]
Five heat-forged simulated products were prepared for each test number. Drill drilling was performed at an arbitrary position on the prepared five thermal forging simulated products, and the amount of tool wear when a total of 50 holes were drilled was measured. The drill diameter was 10 mm, and the rotation speed of the main shaft was 1000 times / min.

  The case where the tool wear amount was 0 to 10 μm was evaluated as “S”, the case where 11 to 30 μm was evaluated as “A”, the case where 31 to 50 μm was evaluated as “B”, and the case where 51 to 70 μm was evaluated as “C”. The case where the amount of tool wear was 71 μm or more was evaluated as “x”.

  In the case of evaluations “S” and “A” to “C”, it was determined that sufficient machinability was obtained. In the case of evaluation “x”, it was judged that the machinability was low.

[Evaluation results]
The evaluation results are shown in Table 2. Referring to Table 2, the chemical compositions of test numbers E-1 to E-45 were appropriate, and fn1 also satisfied the formula (1). Furthermore, the degree of superheat ΔT and the casting speed Vc were also appropriate. Therefore, the number density of coarse TiN was in the range of 0.3 to 4.0 pieces / mm 2. As a result, an excellent cracking property was obtained although the area ratio of bainite was 0 to 30%. Furthermore, the yield strength YS, fatigue strength, machinability, and hot workability were also good.

  On the other hand, the V content of test number C-1 was too high. Therefore, the strength was too high and the machinability was low.

  The V content of test number C-2 was too low. Therefore, the fatigue strength was low.

  The Ti content of test number C-3 was too high. Therefore, hot workability was low.

  The Ti content of test number C-4 was too low. Therefore, the fatigue strength was low. Furthermore, the degree of superheat ΔT was too small. Therefore, the number density of coarse TiN was low. As a result, the cracking property in the steel material containing bainite was low.

  The N content of test number C-5 was too high. Therefore, hot workability was low.

  The N content of test number C-6 was low, and the number density of coarse TiN was low. Therefore, the cracking property in the steel material containing bainite was low.

  In test number C-7, fn1 was too high. Therefore, machinability was low.

  In test number C-8, fn1 was too low. Therefore, the yield strength was low.

  In test number C-9, the chemical composition was appropriate and the formula (1) was satisfied, but the degree of superheat ΔT was too large. Therefore, the number density of coarse TiN was too high. As a result, the hot workability was low.

  In test number C-10, the chemical composition was appropriate, and although the formula (1) was satisfied, the degree of superheat ΔT was too small. Therefore, the number density of coarse TiN was too low. As a result, the cracking property in the steel material containing bainite was low.

  The chemical composition of test number C-11 corresponded to Example 11 of Patent Document 1. In test number C-11, the C content and the Mn content were too low. Therefore, the fatigue strength was low. Furthermore, the N content was too high. Therefore, hot workability was low. Furthermore, the degree of superheat ΔT was too small. Therefore, the number density of coarse TiN was too low. As a result, the cracking property in the steel material containing bainite was low.

  In test number C-12, the chemical composition was appropriate, and although the formula (1) was satisfied, the casting speed Vc was too low. Therefore, the number density of coarse TiN was too high. As a result, the hot workability was low.

  In test number C-13, the chemical composition was appropriate, and although the formula (1) was satisfied, the casting speed Vc was too high. Therefore, the number density of coarse TiN was too low. As a result, the cracking property in the steel material containing bainite was low.

  The embodiment of the present invention has been described above. However, the above-described embodiment is merely an example for carrying out the present invention. Therefore, the present invention is not limited to the above-described embodiment, and can be implemented by appropriately changing the above-described embodiment without departing from the spirit thereof.

Claims (3)

% By mass
C: 0.39 to 0.55%,
Si: 0.10 to 1.00%,
Mn: 0.50 to 1.50%,
P: 0.010-0.100%
S: 0.040 to 0.130%,
Cr: 0.05 to 0.50%,
V: 0.05 to 0.40%,
Ti: 0.15-0.25%,
Al: 0.005 to 0.050%,
N: 0.002 to 0.020%,
Cu: 0 to 0.40%,
Ni: 0 to 0.30%,
Mo: 0 to 0.10%,
Pb: 0 to 0.30%,
Te: 0 to 0.3000%,
Ca: 0 to 0.0100%, and
Bi: 0 to 0.3000% is contained, the balance is Fe and impurities, and has a chemical composition satisfying the formula (1),
A non-heat treated steel bar in which the number density of TiN having an equivalent circle diameter of 20 μm or more in the steel is 0.3 to 4.0 pieces / mm ² .
0.60 ≦ C + 0.2Mn + 0.25Cr + 0.75V + 0.81Mo ≦ 1.00 (1)
Here, the content (mass%) of the corresponding element is substituted for the element symbol in the formula (1). The non-heat treated steel bar according to claim 1,
The chemical composition is
Cu: 0.01-0.40%,
Ni: 0.01 to 0.30%, and
Mo: Non-tempered steel bar containing one or more selected from the group consisting of 0.01 to 0.10%. The non-heat treated steel bar according to claim 1 or claim 2,
The chemical composition is
Pb: 0.05 to 0.30%,
Te: 0.0003 to 0.3000%,
Ca: 0.0003 to 0.0100%, and
Bi: Non-tempered steel bar containing one or more selected from the group consisting of 0.0003 to 0.3000%.

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